Effects of Ac150 on virulence and pathogenesis of Autographa californica multiple nucleopolyhedrovirus in noctuid hosts.

Ac150 is expressed late during infection of cultured lepidopteran insect cells by Autographa californica multiple nucleopolyhedrovirus. The Ac150 gene product is predicted to have a molecular mass of 11 161 Da and consists of a hydrophobic N terminus and a single 'peritrophin-A'-like domain, connected by a short region of charged amino acids. An Ac150 deletion mutant and its parental wild-type virus were compared for differences in virulence by both oral and intrahaemocoelic routes of infection. It was found that the mutant was significantly less virulent in larvae of all three host species tested (Heliothis virescens, Spodoptera exigua and Trichoplusia ni) when occlusions were administered orally, but not when isolated occlusion-derived virus (ODV) was administered orally or budded virus was administered intrahaemocoelically. ODV yields were the same from equal numbers of mutant and wild-type occlusions, and nucleocapsid-distribution frequencies within the two ODV populations were the same, eliminating these features as explanations for the observed differences in virulence. Comparison of pathogenesis, as revealed by lacZ expression from identical reporter-gene cassettes in the mutant and wild-type virus, indicated that the mutant was less efficient at establishing primary infection in midgut cells; otherwise, it exhibited infection kinetics identical to those of wild-type virus. Ac150, therefore, can be considered a per os infection factor that mediates, but is not essential for, oral infection.